The present invention relates to a corrosion resistant primer and, more particularly, a waterborne epoxy corrosion resistant primer which is free of hexavalent chromium compounds.
Structural alloys, including alloys of aluminum, steel, etc, are commonly used in aerospace, marine, auto and other applications. Generally, these alloys are protected against corrosion by applying primers or organic coatings containing corrosion inhibitors. Typically, the corrosion resistant primers used for the structural alloys are solvent based epoxy primers which employ hexavalent chromium as an active corrosion inhibitor pigment additive. These primers rely on the hexavalent chromium for corrosion protection. All forms of hexavalent chromium are recognized by the United States National Institute of Environmental Health Sciences as a Group I known human carcinogen. In addition, again with the enactment of federal and state legislation to reduce hexavalent chromium volatile organic content (VOC) levels in paints and primers, considerable attention has been given to waterborne and/or water-reducible resin binders. Accordingly, the use of corrosion inhibiting compounds which contain forms of hexavalent chromium are subject to stringent regulation and control. It would be very beneficial to eliminate hexavalent chromium as a corrosion inhibiting additive to such primers.
Accordingly, it is the principle object of the present invention to provide a corrosion resistant primer which is free of hexavalent chromium.
It is a particular object of the present invention to provide a waterborne epoxy corrosion resistant primer as set forth above which is effective in preventing attack on structural metal alloys.
It is a still further object of the present invention to provide a corrosion inhibiting waterborne epoxy primer as set forth above which is effective against both general corrosion and pitting corrosion.
Further objects and advantages of the present invention will appear hereinbelow.
In accordance with the present invention the foregoing objects and advantages are readily obtained.
The present invention is drawn to a waterborne epoxy corrosion resistant primer comprising a waterborne epoxy, a curing agent, and a non-chromate corrosion inhibiting pigment. In accordance with the present invention, the non-chromate corrosion inhibiting pigment comprises a non-chromate corrosion inhibiting pigment additive in combination with other pigment extenders and the like. The non-chromate corrosion inhibiting pigment additive is selected from the group consisting of cerous molybdate with bismuth vanadate, cerous molybdate with strontium tungstate, cerous phosphate with strontium tungstate, bismuth vanadate with bismuth molybdate and strontium tungstate, and mixtures thereof. The particularly preferred non-chromate corrosion inhibiting primer additive is cerous molybdate with strontium tungstate.
The present invention is drawn to a chromate free waterborne epoxy corrosion resistant primer comprising a waterborne epoxy, a curing agent, and a non-chromate corrosion inhibiting additive.
In accordance with the present invention, the non-chromate corrosion inhibiting pigment comprises (1) a non-chromate corrosion inhibiting additive selected from the group consisting of cerous molybdate with bismuth vanadate, cerous molybdate with strontium tungstate, cerous phosphate with strontium tungstate, bismuth vanadate with bismuth molybdate and strontium tungstate, and mixtures thereof, and (2) extender pigments selected from the group consisting of titanium dioxide, clay or aluminum silicate, calcium carbonate, talc or magnesium silicate, zinc oxide, and barytes or barium sulfate. The non-chromate corrosion inhibiting pigment (that is, the total pigment which includes the non-chromate corrosion-inhibiting additive and extender pigments)is present in an amount of between 25 to 60 wt. %, preferably 40 to 55 wt. %, with respect to the final primer composition. The non-chromate corrosion inhibiting additive comprises between 5 to 40 wt. %, preferably 25 to 35 wt. %, with respect to the total pigment composition.
In a preferred embodiment of the present invention, the waterborne epoxy corrosion resistant primer comprises a waterborne epoxy selected from the group consisting of waterborne non-volatile dispersion of diglycidyl ether bisphenol A modified epoxies having weight per epoxide (WPE) value between 450-700 and viscosity between 12000-19000 cps. In one embodiment of the present invention, the dispersion is based on a medium molecular weight wherein the dispersion is based on a medium molecular weight, polyfunctional resin dispersion, which on curing forms a high crosslink density polymer having very good hardness and resistance. In a further embodiment, the dispersion is based on an aqueous dispersion of a semi-solid standard bisphenol A epoxy resin of high molecular weight epoxy resin. The waterborne epoxy is present in an amount of 5 to 50 wt. %, preferably between 15 to 30 wt. % with respect to the final primer composition.
In accordance with the present invention, the curing agent is selected from the group consisting of waterborne or water-reducible modified amine or polyamidoamine adducts having amine value between 300-450. In a preferred embodiment of the present invention, the curing agent further includes a hydrophobic curing agent to impact water in corrosion resistance. The curing agent is present in an amount of between 2 to 20 wt. %, preferably 5 to 15 wt. %, with respect to the final primer composition.
The chromate free waterborne epoxy corrosion resistant primer of the present invention is particularly useful in preventing general corrosion and pitting corrosion on metal substrates, particularly, alloys of aluminum, alloys of steel, stainless steels, high strength stainless steel alloys, and the like. The use of the chromate free waterborne epoxy of the present invention is useful in all industries including the aerospace industry, automotive industry, architectural industry, packaging industry, electronics industry, HBAC and marine.
The corrosion inhibiting properties of the chromate free waterborne epoxy corrosion resistant primer of the present invention will be made clear from the following example.